The purpose of this amended RO1 proposal is to re-establish funding for the research supported by our recently completed 2-year exploratory research grant (R21 AG19365) that was responsive to PA-00-056 "Skeletal Muscle Perfusion, Aging, and Cardiovascular Disease". Our R21 award work demonstrated that: 1) basal whole-leg blood flow (LBF) and vascular conductance (LVC) are 25-30% lower in men and women 65 years of age compared with young adult controls; and 2) the decreases in LBF and LVC are associated with increased tonic sympathetic nervous system (SNS) alpha-adrenergic vasoconstriction and reduced nitric oxide (NO) vasodilatory tone. Among postmenopausal women, however, those taking hormone replacement therapy (HRT) had greater LBF and LVC than their estrogen deficient peers. The primary working hypothesis of the present proposal is that HRT (transdermal estradiol) and/or a selective estrogen receptor modulator (SERM; raloxifene) will increase LBF in healthy previously estrogen deficient postmenopausal women by increasing LVC, with or without increasing systemic BF (cardiac output). Secondary hypotheses are: 1) the increases in LVC with HRT and SERM administration will be associated with an increased local vasodilatory state (increased NO bioavailability/reduced oxidative stress; decreased endothelin-1 lET-1] concentrations), and either unchanged or reduced SNS activity; and 2) the increases in LBF and LVC in response to HRT and a SERM will be greatest in women homozygous for the C allele of the estrogen receptor (ER)-a gene IVSI 401 polymorphism and least in women who are homozygous for the T allele. LBF, arterial blood pressure, LVC, cardiac output, muscle sympathetic nerve activity, plasma catecholamine concentrations, leg peak reactive hyperemia and VC, measures of local vasodilatory/vasoconstrictor influences (including flow-mediated dilation and vascular oxidative stress), and ER-alpha gene IVS1401 polymorphisms will be determined in healthy postmenopausal females before and during 2, 6, and 12 weeks of either HRT, SERM administration, or placebo control (randomized, double blind). Insight into the molecular mechanisms involved will be provided by a novel translational research approach in which eNOS, ET-1, and other protein expressions of interest will be determined from vascular endothelial cell "biopsies". All experimental protocols will be conducted in the UC-Boulder General Clinical Research Center. The results should provide new insight into the effects of HRT and a SERM for augmenting LBF and LVC in estrogen deficient postmenopausal women and the underlying physiological mechanisms.